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First Planet Orbiting Twin Suns Discovered

Two hundred light-years away, a Saturn-size world called Kepler-16b orbits two stars. The astronomers who spotted this planet, the first ever seen orbiting both stars of a two-star system, released their finding to the world today in a study published in Science.

Astronomers looked in the right place at the right time and saw something that had only been seen in science fiction: a planet orbiting the two suns of its alien solar system. Scientists using the planet-hunting Kepler telescope spotted the Saturn-size world in a star system they call Kepler-16, and today they reveal their discovery in the journal Science. It's the first exoplanet ever spotted orbiting both stars in a binary star system, evoking images of Star Wars' Luke Skywalker gazing into the twin sunset on Tatooine.

The Kepler-16 suns are both smaller than ours: One, orange in color, is 69 percent as massive as our sun, and the other, which appears red, is 20 percent as massive. The two stars orbit each other every 41 days. Life on a two-sun planet is difficult to imagine, says Nader Haghighipour, an astronomer at the NASA Astrobiology Institute at the University of Hawaii at Manoa. Sometimes, both stars would have set, and there would be total darkness on the planet. Other times, one sun would be up, and then the other would rise, turning things extremely bright. "The concept of day and night, dark and light, would have totally different meanings," Haghighipour, who was not involved in the study, says.

Our own single-star system is actually an oddball; most solar systems have two or more stars. But until now, astronomers have had to exclude this enormous group of star systems from the search for Earth-like planets. That's because one of the primary ways astronomers search for exoplanets is through the "wobble" method—faraway planets are so hard to see, scientists instead look to stars for a telltale wobble that could be evidence of a planet's gravitational pull. When there are two stars, the two enormous masses pull on each other and mask the signatures of planets. In this case, though, astronomers got lucky: They caught sight of the Kepler-16 planet as it passed directly between the telescope and its two suns. It's the first direct evidence of a planet orbiting two stars, according to study lead author Laurance Doyle. "It's a whole new kind of planetary system," he says.

The discovery happened only because the planet and both its suns were lined up on a plane with Earth, so that the scientists were at the right angle to see the planet as it crossed its suns. Of the 150,000 stars the Kepler spacecraft is watching, scientists chose a few thousand potential two-star systems to keep an eye on. It just so happened that the planet and the two stars lined up with the telescope's line of sight. "The team had to be smart to choose their initial targets properly, then keep their fingers crossed," Haghighipour says.

In the past two years, Haghighipour says, there have been a couple of other announcements of planets orbiting two stars, but they turned out to be false claims. "This one is for real," he says. "It's a very important and profound discovery."

Even if we could travel the 200 light-years to Kepler-16b, it would be no vacation destination. The planet's temperature falls between about minus 70 to minus 100 degrees Fahrenheit. At its warmest, Doyle says, the planet is comparable to "a nippy winter in Antarctica." Plus, the newly discovered planet is about the size of Saturn (95 times more massive than Earth), and like our solar system's sixth planet, it would have a thick, gaseous atmosphere.

As a result, the Kepler-16 planet probably isn't a good place to look for life. But there is one hope for life-hunters, Doyle says: "If it has a moon, it has a shot." Planets of Kepler-16's size can accommodate moons as big as Earth. If the moon were large enough to hold an atmosphere, Haghighipour says, that could allow the greenhouse effect to take hold and even out the temperatures across the moon. (Saturn's moon Titan, for example, has a thick atmosphere—though it's made of compounds like methane and ethane that aren't terribly hospitable to life as we know it.) If the hypothetical moon were too big, however, it would attract a lot of gas and become too hot; plus, it would exert too much pressure on its core for it to have continental plates, which help to cycle carbon dioxide on Earth and make life possible. The right moon for life would be no smaller than half the size of Earth and no bigger than about three Earths, says Haghighipour. Doyle and his team are searching for this moon, but so far have been unsuccessful.

Whether or not a moon is found, the discovery of this planet means that scientists can now begin to search for other planets orbiting two stars by using the same methods. Doyle is confident that there are many similar planets out there. "These are rare in the galaxy, but it's a big galaxy," he says. "Back of the envelope, I'd say there are two million more."

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